Move pre, postinstall, scenario, and apps to stack

Preinstall, postinstall, scenario, and apps are specific
to stack composition and more appropriate to locate them
within the stacks themselves rather than the installers.
This makes it possible for different stacks to configure
target deployment according to their needs and use the
installer without thinking about potential effects the
changes could cause on other stacks that may be using the
same installer.

An example to this is Kubernetes and ONAP stacks and the
CEPH configuration. Both stacks use Kubespray but the
configuration of Kubernetes differs between them. By moving
the pre/post/scenario/app configuration into stacks
themselves, they become independent from each other as
much as possible.

Please note that once this change goes in, the dependent
change needs to be verified and submitted in order to
conclude the transition.

https://gerrit.nordix.org/c/infra/installer/kubespray/+/5157

Change-Id: I24d7e9546034b385565708f528ed9caddf03ae26